Method and apparatus for continuously transforming plutonium oxalate into plutonium oxide

ABSTRACT

The present invention relates to a method of transforming plutonium oxalate into plutonium oxide by drying and then calcining. In said method, in a characteristic manner, the operations of drying and calcining are implemented continuously, in the presence of oxygen and with gas extraction, in two adjacent zones of a single apparatus, e.g. of the screw oven type, that is maintained at negative pressure. The present invention also provides apparatus suitable for implementing said method.

[0001] The present invention relates to a method and apparatus forcontinuously transforming plutonium oxalate into plutonium oxide. Moreprecisely, the invention relates to a method and to an apparatussuitable for continuously transforming plutonium oxalate into plutoniumoxide by drying and then calcining said oxalate, in the presence ofoxygen.

BACKGROUND OF THE INVENTION

[0002] The invention lies in the general context of recoveringplutonium. Such recovery of plutonium, starting from purified plutoniumnitrate solutions themselves coming from extraction cycles,conventionally comprises four steps:

[0003] the plutonium is precipitated from said solutions in oxalateform;

[0004] the resulting precipitate is recovered by filtering and is thendrained;

[0005] it is dried and then calcined; and

[0006] it is finally homogenized.

[0007] The plutonium oxide powder obtained in this way is then packagedin compliance with the safety rules in force so as to protect man andthe environment.

[0008] The operations of drying and calcining the drained plutoniumoxalate (said drained plutonium oxalate generally still containing about30% by weight water, bonded water+interstitial water) are performeddiscontinuously (in batches) in the prior art in distinct pieces ofequipment, always under double containment for obvious safety reasons.Conventional ovens housed in glove boxes are used in succession andcompletely independently of each other: the first for drying saiddrained plutonium oxalate and the second for calcining the dried oxalatein the presence of oxygen.

OBJECTS AND SUMMARY OF THE INVENTION

[0009] The Applicant now proposes an improvement to that technique oftransforming plutonium oxalate into plutonium oxide by drying and thencalcining. It proposes implementing said transformation continuously, ina single apparatus.

[0010] In a first aspect, the present invention thus provides a methodof transforming plutonium oxalate into plutonium oxide by drying andthen calcining. In characteristic manner, in the context of saidmethod., said operations of drying and calcining are implementedcontinuously in the presence of oxygen and with gas extraction in twoadjacent zones of a single apparatus maintained at negative pressure(the said—negative—pressure is the relative pressure with regard to theambient pressure).

[0011] In the invention, a single device is used, e.g. of the screw ovenor conveyor oven type, that is maintained at negative pressure (forcontainment problems) in order to treat plutonium oxalate continuouslyboth to dry and to calcine continuously said plutonium oxalate. Thesingle apparatus is advantageously of the screw oven type as describedin general terms below in the present specification and in more detailedmanner, likewise below in the present specification, with reference tothe accompanying figures. For reasons of double containment, said singleapparatus is normally disposed in a glove box.

[0012] Within said single apparatus, the treated oxalate precipitateprogresses continuously from a first zone in which it is dried towards asecond zone in which it is calcined.

[0013] Oxygen is used within said single apparatus to transform saidoxalate precipitate into oxide.

[0014] The drying and calcining of said precipitated oxalate generatesgas continuously, in particular water vapor and carbon dioxide. Said gasmust be extracted specifically to ensure that the single apparatuswithin which the method is implemented is maintained at negativepressure.

[0015] The gas is advantageously extracted under conditions optimized sothat gas extraction has minimal impact on the drying and calciningprocess. In particular, the amount of powder entrained is minimized asis any remoistening of dried powder by the water that is generatedduring drying . . . . For this purpose, said gas extraction isadvantageously implemented in the drying zone of the sole apparatuswithin which the method is implemented. It is preferably implemented insaid zone as far upstream as possible (although it must clearly beimplemented downstream from the zone where the load to be treated isintroduced).

[0016] In the context of the invention, as in the prior art, theextracted gas is charged with radioactive dust. Prior to beingevacuated, said gas must therefore necessarily be dedusted. For thispurpose, it is possible to use prior art techniques, and in particular:

[0017] said gases can be treated in a condenser-washer; and/or

[0018] they can be caused to pass through a filter element.

[0019] Such a filter element is placed on the gas extraction circuit. Asa general rule n parallel-connected elements of this type are used.Conventionally, such an element is constituted by a filter mediumdisposed in a support and referred to as a filtering cartridge. Whateverthe context of use, once the filter medium has become clogged, such acartridge is no longer effective. In ordinary use, such cartridges needto be changed regularly. Clogging means that they should be replacedsystematically, which requires the gas filtering process in question tobe interrupted . . . .

[0020] When implementing the method of the invention, the extracted gas(advantageously extracted from the drying zone) is advantageouslycleaned of radioactive dust by being filtered through at least onefiltering cartridge.

[0021] In the context of the present invention, it is recommended toimplement said filtering under the particularly advantageous conditionsof implementing said filtering without interruption and with in situregeneration of the clogged filter medium. The method of the inventionis thus advantageously implemented with the extracted gas being filteredthrough at least one filtering cartridge that contains a non-deformablefilter medium; said filter medium is unclogged intermittently, in situ,without interrupting the filtering. Such unclogging is made possible bythe filter medium in question being undeformable. To unclog the medium,it is recommended to puff a small pulse of gas in the reverse direction(relative to the filtered gas) through the filter medium in question.

[0022] Said quantity of puffed gas is clearly limited. Under nocircumstances is a genuine back-flow generated; that would implyintroducing a non-negligible quantity of gas into the apparatus, raisingthe pressure within said apparatus (which it is recalled mustnecessarily be at negative pressure), and could even cause dust to beblown upstream therefrom. Said quantity of gas is advantageouslystrictly limited to the quantity required for obtaining the desiredpulse effect, which pulse effect is suitable for obtaining the intendedunclogging.

[0023] Since the atmosphere of the apparatus is thus not disturbed (evenfor a brief interval), filtering can be carried out withoutinterruption. Clearly, the greater the overall volume of the apparatus,the greater the pulse of gas that can be admitted. Similarly, it isappropriate to increase the number of filtering cartridges used inapparatus of small volume. Regenerating such cartridges one by one thenrequires only very small quantities of gas in each puff . . . .

[0024] The looked-for pulse effect is optimized if the small pulse ofgas is puffed only when a genuine layer or “cake” of dust has formed onthe filter medium to be regenerated. Under such optimum conditions ofimplementation—i.e. when there is a layer of dust of perceptiblethickness on the filter medium—the pulse of gas is subjected to maximumpressure drop and is certain not to generate any back-flow within theapparatus. In a preferred variant implementation of the uncloggingmethod, it is therefore strongly recommended to puff a small pulse ofgas only once a layer of dust has formed on the filter medium. If thegas is puffed in prematurely, two undesirable phenomena can be observed:

[0025] the filter medium in question becomes reclogged very quickly; and

[0026] a back-flow is observed within the apparatus; the puffed gas hasnot been subjected to sufficient pressure drop.

[0027] The person skilled in the art knows how to minimize and evenavoid these undesirable phenomena; and in any event knows how todetermine the minimum or optimum thickness of cake for obtaining bestresults, given the various parameters in question (the internal volumeof the filter medium to be regenerated, the total volume of theapparatus, . . .).

[0028] It is recalled at this point that filter media that can beunclogged in situ are undeformable. It is therefore clear that saidfilter media are unclogged in the invention without deforming them orvibrating them (advantageously by breaking off the genuine cake that hasformed on them).

[0029] Such undeformable filter media are familiar to the person skilledin the art. They can be constituted in particular by filter media madeof sintered metal, of woven metal, or of ceramics.

[0030] The small pulse of gas is advantageously blown by means of anozzle, advantageously a nozzle having a sonic throat. The profile ofsuch a nozzle should be optimized to obtain the looked-for pulse effect.Such optimization is within the competence of the person skilled in theart.

[0031] The gas thus puffed into the gas extraction zone must clearlyavoid disturbing the on-going drying and calcining process, whether interms of quantity (see above) and/or in terms of kind. In particular, itis recommended to puff air or an inert gas, and in particular to puffair. Unclogging can thus be performed at lower cost, withoutsignificantly affecting the oxidizing atmosphere of the apparatus.

[0032] In the light of the above, the person skilled in the art can seethe advantage of the present invention. The treatment (drying+calcining)of the (moist) plutonium oxalate is performed continuously within asingle apparatus and is therefore much more efficient than the sametreatment as implemented in the prior art as two distinct stages(drying, calcining), each being implemented within a different piece ofapparatus.

[0033] In its second aspect, the invention provides apparatus suitablefor implementing the above-described method. Within said singleapparatus, the oxalate precipitate is successively dried and thencalcined.

[0034] The apparatus as described below is particularly preferred.

[0035] It is mentioned at this point that the method of the invention isentirely suitable for being implemented in apparatuses that are somewhatdifferent . . . .

[0036] The particularly preferred apparatus comprises:

[0037] an oven of generally cylindrical shape fitted on its horizontallongitudinal axis with an Archimedes' type screw, said screw on rotatingbeing suitable for transferring heat-treated substance from a first zoneof said oven where it is dried to a second zone where it is calcined;the body of said oven presenting firstly, suitable openings for:

[0038] feeding the oven with plutonium oxalate at one of its ends,adjacent to the first zone;

[0039] emptying plutonium oxide from the oven at its other end, adjacentto the second zone;

[0040] injecting oxygen into said oven; and

[0041] extracting gas from said oven;

[0042] and presenting secondly, in its thickness, housings for receivingheater elements; said elements being suitable for being regulated todefine temperature conditions inside said oven that are suitable forimplementing drying in said first zone and calcining in said secondzone;

[0043] gas extraction means suitable for extracting gas from said oven,while keeping the oven at negative pressure; said means actingdownstream from the gas extraction opening formed in the body of saidoven; and advantageously

[0044] removal means suitable for removing entrained dust from said gasextracted through said extraction opening by said gas extraction means;said removal means advantageously being constituted by a filterassembly.

[0045] The heater elements for positioning in the thickness of the bodyof said oven (in housings provided for this purpose) are, in a preferredvariant, essentially constituted by heater rods of appropriate length.Said heater rods for positioning longitudinally relative to the axis ofthe oven are advantageously associated with means for regulating theheating power they evolve, e.g. thermocouples.

[0046] In a particularly preferred variant, two types of such heater rodare used in the body of the oven (in housings provided for thispurpose):

[0047] at least one “short” rod which is responsible for part of thedrying of the plutonium oxalate in the first zone of the oven; and

[0048] at least one “long” rod which is responsible both for theremaining portion of the drying of the plutonium oxalate in said firstzone of the oven (in addition to said short rod(s)) and for calciningsaid plutonium oxalate in the second zone of the oven.

[0049] It is mentioned above that gas extraction advantageously takesplace in the drying zone. Thus, the gas extraction opening isadvantageously provided in the drying zone (first zone of the oven),naturally downstream from the opening for feeding the oven withplutonium oxalate.

[0050] The means for removing entrained dust from the extracted gas canconsist in particular in a condenser-washer and/or a filter assembly.Said means are advantageously constituted by a filter assembly. Such afilter assembly generally comprises at least one filtering cartridge,and advantageously a set of n filtering cartridges connected inparallel.

[0051] Advantageously, in order to implement unclogging in situ, asdescribed above, use is made of a one or more filtering cartridgescontaining undeformable filter medium and each of said cartridges isassociated with means for unclogging its filter medium in situ.

[0052] Said unclogging means generally comprise a nozzle suitable forintermittently passing a small pulse of gas into the clogged filtermedium.

[0053] The pulse effect is advantageously obtained by feeding saidnozzle with said gas under the control of a fast opening/closingsolenoid valve.

[0054] The gas used for unclogging can be taken from an appropriatenetwork. It can also be taken from a supply or tank provided for thispurpose, in which case after being inserted into said tank, the gas isstored therein under pressure. Said tank is naturally suitable for beingisolated by stop cocks. In such a tank, the gas pressure is adjusted bymeans of a pressure-reducing valve. Downstream from the tank, the gascan be injected into the filter medium that is to be regenerated.Injection is performed by means of the nozzle whose profile and positionin the system are advantageously optimized. The flow speed achieved inthe constricted portion of said nozzle can be sonic. Puffing isadvantageously under the control of a solenoid valve.

[0055] The mass flow rate of the gas puffed during a pulse can thus befully controlled.

[0056] In characteristic manner, a single oven is used in the apparatusof the invention, within which oven (drained) plutonium oxalate issuccessively dried and then calcined.

[0057] Said single oven presents the four openings listed above.Advantageously, the said four openings are arranged as follows:

[0058] the oxalate feed opening is arranged in the top portion of theoven body;

[0059] the gas extraction opening is likewise arranged in the topportion of the oven body, downstream from said feed opening. Asmentioned above, it is advantageous for said extraction opening to beclose to said feed opening;

[0060] the emptying opening is arranged in the bottom portion of thebody of said oven; and

[0061] the oxygen injection opening is arranged in the top portion ofthe body of said oven, advantageously on the same axis as said emptyingopening;

[0062] and naturally the feed opening and the gas extraction opening arein the first zone for drying, while the emptying opening and the oxygeninjection opening are in the second zone for calcining.

[0063] It will readily be understood that in order to maximize theresidence time of the powder in an oven of given length, in order tomaximize the time during which said powder is in contact with theoxygen, the oxygen injection opening and the emptying opening arepositioned as far away as possible from the feed opening and the gasextraction opening.

[0064] The structure of said oven is advantageously as described below.The body of said oven is closed in leakproof manner by a front bearing(feed end) and by a rear bearing (emptying end), with the Archimedes'type screw being engaged between said two bearings. The drive device(motor) for said screw is arranged upstream of said front bearing and isadvantageously coupled for safety reasons to an assembly for detectingthat said screw is indeed rotating. At the rear bearing, provision ismade for it to be possible to inject a sweeping gas. The use of such agas—air or inert gas such as nitrogen, as a general rule—can beadvantageous. In any event, said gas cools said rear bearing andreinforces containment.

[0065] The oven body—the main structural element of the apparatus of theinvention which contains the heater elements—is advantageously lagged ina thermal insulation assembly. Said assembly is advantageously protectedby a case fitted with bellows to compensate for expansion. Said case isof a shape that matches that of the oven. It is therefore generallycylindrical in shape.

[0066] It is recalled at this point that for reasons of doublecontainment, the apparatus of the invention is generally implementedinside a glove box. In such a context, the housings provided in the bodyof the oven for the heater elements are advantageously open to theoutside of said glove box so that said heater elements are accessiblefrom outside said glove box. It will then be understood that the frontface of said oven body advantageously constitutes a portion of the frontface of said glove box.

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] An advantageous variant of the apparatus of the invention isdescribed below with reference to accompanying FIG. 1. FIG. 1 is adiagrammatic longitudinal section showing apparatus constituting saidadvantageous variant of the invention.

[0068]FIG. 2 is a diagram showing the positions and the types of heatingrods used.

MORE DETAILED DESCRIPTION

[0069] As shown diagrammatically in FIG. 1, the apparatus of theinvention comprises:

[0070] an oven body 1 that is generally cylindrical in shape, comprisingan opening 11 at one of its ends, in the top portion thereof, enablingit to be fed with plutonium oxalate, and an opening 12 at the other endof said oven body, in the bottom portion thereof, enabling it to beemptied. A third opening 9 is provided in the top portion on the sameaxis as the emptying opening 12 for injecting oxygen. A fourth opening15 is provided to enable gas to be extracted from said oven body 1;

[0071] an Archimedes' type screw 2 placed inside said oven body 1 andengaged in a rigid front bearing 5 at the inlet end. Said screw 2 isrotated by a motor and gear box unit 7 associated with a torque meterand a ball torque limiter mounted at the end of the bearing 5. Anassembly 18 for detecting rotation makes it possible to verify that saidscrew 2 is turning. For obvious reasons of ease of maintenance, thesystem for driving said screw 2 is located outside the glove box 20 inwhich the apparatus of the invention as a whole is disposed. The bearing5 is isolated from the atmosphere inside the oven by an axial leakproofgraphite/metal sealing gasket. Said front bearing 5 and said screw 2 canneed changing during the lifetime of the oven and are thereforeremovable;

[0072] a rear bearing 6 constituted by a graphite ring enabling saidscrew 2 to rotate in said oven body 1. Said rear bearing 6 enablessweeping air 10 to be injected into said oven body 1. Said sweeping aircools said rear bearing 6. Containment between said oven and the glovebox 20 in which it is disposed is obtained dynamically by means of saidsweeping air;

[0073] a set of heater rods 4 (Joule effect heaters), fitted withrespective thermocouples (to regulate said heating), and comprising atleast one long rod for both drying and calcining, and at least one shortrod for drying only. The positioning and the temperatures of said heaterrods are important, since they determine the temperature profile insidethe oven 1 and thus determine the quality of the final oxide. Onepossible disposition of such rods 4 is shown in FIG. 2. References A andB designate two drying and calcining rods, while references C, D, E, andF designate four drying rods. The heating and regulating elements canneed changing during the lifetime of the oven so they are removable.They are inserted from the front face of the glove box 20 in which theapparatus of the invention is located. They can thus be replaced withoutcontamination;

[0074] a thermal insulation assembly 3 around said oven body 1. Thethermal insulation is protected by a cylindrical case fitted withexpansion-compensating bellows 8. The essential need for access to therear bearing 6 means that said thermal insulation 3 must be easilyremoved from the glove holes in the glove box 20; and

[0075] a gas filtering assembly F made up of sintered stainless steelcartridges 13 and means 14 for unclogging said cartridges 13. Gas isextracted under the action of means P from the drying portion of theoven (first zone) by passing through a funnel 15. Extraction isperformed under conditions which minimize entrainment of powder. Saidfilter assembly F is heated by a heating collar 16 so as to avoid anycondensation in the filtering cartridges 13. Said filter assembly F issurrounded by thermal insulation 17. The cartridges 13 which might needto be replaced are easily removable.

[0076] Drained oxalate, generally coming from a rotary filter, fallsunder gravity via 11 into the top end of the oven body 1 and it leavesthe oven body in the form of plutonium oxide from the bottom of theother end at 12. The substance is transferred horizontally by theArchimedes' screw 2 which turns continuously inside said oven body 1.Throughout its transfer, the temperature of the substance is raised.

[0077] The heating and regulating elements 4 (resistances 4 andthermocouples not shown) are housed in the thickness of the oven body 1.

[0078] The gas generated by drying and calcining the oxalate, the excessoxygen, and the sweeping air are entrained by the means P towards thefilter assembly F which includes the filter cartridges 13. The entrainedpowder is retained in the filter media of said cartridges 13. Saidfilter media are unclogged in situ by the unclogging means 14.

[0079] In situ unclogging is performed by sequentially delivering a verysmall quantity of air at the speed of sound into each of the cartridges13, the air being taken from a supply under pressure. The installationas a whole naturally remains at negative pressure (at a pressure belowthe ambient pressure). The caked powder falls directly into the oven.This original unclogging can be referred to as being performed by a“bang” effect.

[0080] In order to leave the oven free to expand when raised intemperature, said oven body 1 rests via cradles fitted with wheels onrails that are secured to the glove box at the substance inlet end, anda bellows flange connects the front face of the glove box to the frontface of said oven body. Said flange guarantees continuity ofcontainment. These means—cradles, rails, bellows flanges—are not shownin FIG. 1.

[0081] The oven is controlled from a remote control station.

[0082] The equipment is naturally dimensioned to be sub-critical.

[0083] The oven must not be capable of being expelled and it must remainsealed and it must retain its geometrical characteristics even after anearthquake.

[0084] The invention is illustrated by the following example.

[0085] The method of the invention has been implemented in apparatus ofthe invention of the kind shown diagrammatically in FIG. 1.

[0086] The oven was fitted with an Archimedes' type screw. The length ofsaid oven was limited for mechanical reasons associated with the spanbetween the bearings. Its length was about 2 meters (m). The length ofthe screw was 1700 mm. It was rotated in a cylindrical volume (theinside volume of the oven) whose diameter was 162 mm.

[0087] Said oven was used under the following conditions:

[0088] when maximally full:

[0089] at 60% (ratio of volume occupied by the powder over total volumeavailable in the oven), in the drying zone; and

[0090] to a depth of 16 mm in the calcining zone; said drying zonecorresponding to the first third of the length of the oven, and saidcalcining zone corresponded to the last two-thirds of said length;

[0091] for a minimum powder transit time of 45 minutes: 15 minutes inthe drying zone, and 30 minutes in the calcining zone. The speed ofrotation of the screw (said screw being capable of turning at a speedlying in the range 0 to 5 revolutions per minute (rpm)) was in any eventregulated so that the depth of 16 mm of oxide in the calcining zone wasnever exceeded; and

[0092] with heating rods (four drying rods, and two drying and calciningrods) positioned as shown in FIG. 2. Each of said drying rods had atotal length of 650 mm, and more precisely a working length of 395 mmand a power rating of 1.250 kW, while each of the drying and calciningrods had a total length of 1925 mm, and more precisely a working lengthof 1670 mm, and a power rating of 2.4 kW. The total power that said rodscould deliver was therefore 9.8 kW, sufficient for ensuring atemperature gradient in the drying zone from 250° C. to 500° C. and atemperature gradient in the calcining zone of 500° C. to 675° C.

[0093] Said oven was maintained at a negative pressure of about 500 Pa(50 mm of water column) below atmospheric pressure.

[0094] It was fed with 9.9 kg of (moist) plutonium oxalate per hour andtherefore produced 4.5 kg of plutonium oxide per hour (corresponding to4 kg of plutonium per hour, i.e. 96 kg of plutonium per day).

[0095] Said moist plutonium oxalate contained 30% by weight of water(total water=bonded water+interstitial water). Its apparent relativedensity was 0.6. It underwent the following reaction:

Pu(C₂O₄)₂6H₂O+O₂→PuO₂+6H₂O+4CO₂.

[0096] Oxygen was injected into said oven at a rate corresponding totwice the stoichiometric quantity.

[0097] 200 normal liters per hour (Nl/h) of air was also injected viathe rear bearing.

[0098] With reference to the gas filter assembly, the following can bestated. It was constituted by seven Poral® cartridges each having aheight of 250 mm, a diameter of 30 mm, and a filtering area of 300 cm².

[0099] Each of said cartridges was unclogged, in turn, about once every15 minutes, once it had picked up about 12 grams of cake. The pulse ofair delivered for unclogging purposes corresponded to 300 normal cubiccentimeters (Ncm³) of air.

[0100] Said filter assembly filtered 7.8 normal cubic meters per hour(Nm³/h) of gas, corresponding to 5.6 Nm³/h of water vapor, of carbondioxide and of excess oxygen+2.2 Nm³/h of ventilation gas (including theair injected via the rear bearing).

1. A method of transforming plutonium oxalate into plutonium oxide bydrying and then calcining, the method being characterized in that saidoperations of drying and calcining are implemented continuously in thepresence of oxygen with gas extraction in two adjacent zones of a singleapparatus maintained at negative pressure.
 2. The method according toclaim 1, wherein the said single apparatus is a screw oven.
 3. Themethod according to claim 1, wherein said gas is extracted from saidapparatus from its drying zone.
 4. The method according to claim 1,wherein said extracted gas is filtered by passing through at least onefiltering cartridge (13).
 5. The method according to claim 4, whereinsaid filtering cartridge(s) (13) contain(s) undeformable filter medium,and in that said filter medium is intermittently unclogged in situwithout interrupting the filtering.
 6. The method according to claim 5,wherein said unclogging consists in puffing a small pulse of gas as aback-flow through said filter medium.
 7. The method according to claim6, wherein the said small pulse of gas is puffed after a layer hasformed on the said filter medium.
 8. The method according to claim 6,wherein the said small pulse of gas is puffed through a nozzle (14)having a sonic throat.
 9. An apparatus for implementing the method oftransforming plutonium oxalate into plutonium oxide by drying and thencalcining, the apparatus being characterized in that it comprises: anoven of generally cylindrical shape fitted on its horizontallongitudinal axis with an Archimedes' type screw (2), said screw (2) onrotating being suitable for transferring heat-treated substance from afirst zone of said oven where it is dried to a second zone where it iscalcined; the body (1) of said oven presenting firstly, suitableopenings (11, 12, 9, 15) for: feeding the oven with plutonium oxalate atone of its ends, adjacent to the first zone; emptying plutonium oxidefrom the oven at its other end, adjacent to the second zone; injectingoxygen into said oven; and extracting gas from said oven; and providingsecondly, in its thickness, housings for receiving heater elements (4);said elements (4) being suitable for being regulated to definetemperature conditions inside said oven that are suitable forimplementing drying in said first zone and calcining in said secondzone; gas extraction means (P) suitable for extracting gas from saidoven, while keeping the oven at negative pressure; said means (P) actingdownstream from the gas extraction opening (15) formed in the body (1)of said oven; and advantageously removal means suitable for removingentrained dust from said gas extracted through said extraction opening(15) by said gas extraction means (P).
 10. The apparatus according toclaim 9, wherein said removal means are constituted by a filter assembly(F).
 11. The apparatus according to claim 9, wherein said housingsextend longitudinally in the axial direction of the oven and areprovided to receive heater rods.
 12. The apparatus according to claim11, wherein said housings are provided to receive: at least one shortrod for performing part of the drying of plutonium oxalate in said firstzone; and at least one long rod both for performing part of the dryingof plutonium oxalate in said first zone, and for performing thecalcining of the dried plutonium oxalate in said second zone.
 13. Theapparatus according to claim 9, wherein said gas extraction opening (15)is provided in said drying first zone, downstream from the plutoniumoxalate feed opening (11).
 14. The apparatus according to claim 9,wherein it includes a filter assembly (F) which itself comprises atleast one filtering cartridge (13).
 15. The apparatus according to claim14, wherein it includes a filter assembly (F) which comprises a set of nfiltering cartridges (13) connected in parallel.
 16. The apparatusaccording to claim 14, wherein said filtering cartridge(s) (13)contain(s) undeformable filter medium, and in that each filter cartridgeis associated with means for unclogging said filter medium, in situ. 17.The apparatus according to claim 16, wherein said unclogging meanscomprise a nozzle (14) for intermittently puffing a small pulse of gasthrough said filter medium.
 18. The apparatus according to claim 17,wherein said nozzle (14) is fed with said gas under the control of afast opening/closing solenoid valve.
 19. The apparatus according toclaim 9, wherein said feed opening (11) is provided in the top portionof said oven body (1); said gas extraction opening (15) is likewiseprovided in the top portion of said oven body (1), downstream from saidfeed opening (11); said emptying opening (12) is provided in the bottomportion of said oven body; and said oxygen injection opening (9) isprovided in the top portion of said oven body (1).
 20. The apparatusaccording to claim 19, wherein said oxygen injection opening (9) isprovided on the same axis as said emptying opening (12).
 21. Theapparatus according to claim 9, wherein the body (1) of said oven isclosed in sealed manner by a front bearing (5) and by a rear bearing(6), the Archimedes' type screw (2) being engaged in said bearings (5,6); the drive device (7) for driving said screw (2) being disposedupstream from said front bearing (5), with said rear bearing (6) beingarranged to allow sweeping gas (10) to be injected.
 22. The apparatusaccording to claim 21, wherein said drive device (7) is coupled to anassembly (18) for detecting that said screw (2) is actually rotating.23. The apparatus according to claim 9, wherein said body (1) of theoven is lagged in a thermal insulation assembly (3).
 24. The apparatusaccording to claim 23, wherein said thermal insulation assembly (3) isprotected by a case fitted with expansion-compensating bellows (8). 25.The apparatus according to claim 9, wherein it is disposed in a glovebox (20).
 26. The apparatus according to claim 25, wherein the housingsprovided in the body (1) of said oven for said heater elements (4) openout outside said glove box (20).